The conventional propeller fan includes a hub 1 and a plurality of blades 2 attached to the hub 1 as shown in FIGS. 18 and 19. Each blade 2 is formed to be flat as a whole from a leading edge 2a to a trailing edge 2b. Radially outward air flow due to centrifugal force generated by rotation of the fan tends to concentrate air flow to the outer periphery of each blade 2 (refer to Patent Document 1).
This causes the following problems.
(1) The flow pattern on the blade surface of each blade 2 changes depending on the operating state of the propeller fan.
(2) When the operating state of the propeller fan changes, the warpage of each blade 2 and the flow pattern cease according with each other. This degrades the performance of the propeller fan.
Particularly, in the case of a semiopen type propeller fan in which only part of each blade 2 is surrounded by a bellmouth 4 as illustrated in FIGS. 18 and 19, a velocity component in the radial direction of air flow changes significantly in a region on the inlet side of the blade 2.
(3) In the downstream region of the blades 2 surrounded by the bellmouth 4, the state of air flow changes to various forms including a centripetal flow, a flow along the rotation shaft of the fan, and an outward flow.
(4) When the air flow resistance of the propeller fan is great, outward air flow is likely to be generated. Therefore, air flow is concentrated in the outer peripheral region of each blade 2, and the blade 2 does not function effectively in a region in the vicinity of the hub 1.
For the reasons discussed above, the blowing performance of the propeller fan is reduced.
In this regard, a fan has been disclosed in which a plate-like rib is provided on the positive pressure surface of each blade in a radially outer end (blade tip), which is not surrounded by a bellmouth (refer to Patent Document 2). The height of the rib becomes gradually greater from the inlet side toward the outlet side of the blade 2.
However, in a fan having this structure, although leakage vortex flowing from the positive pressure surface to the negative pressure surface of each blade at the radially outer tip is reduced, radially outward air flow caused by the centrifugal force cannot be reduced.
Patent Document 1: International Publication WO2003/072948
Patent Document 2: Japanese Laid-Open Patent Publication No. 5-44695